Organic luminescent diodes or organic light-emitting diodes (OLEDs) are by now generally seen as having the potential to offer an alternative to conventional lighting means such as incandescent lamps or fluorescent lamps within the field of lighting technology. By now, the achieved performance efficiencies (cf. for example D'Andrade et al., Adv. Mater. 16 (2004) 1585) are markedly higher than those of incandescent lamps. Current reports support the conclusion that the efficiencies of fluorescent lamps can also be surpassed. To achieve highest performance efficiencies, phosphorescent emitters are currently used as they are designed in such a way that they can convert 100% of the charge carriers employed into light, i.e. achieve a quantum efficiency of 100%.
Besides the performance efficiency, the service life is another important criterion for the everyday use as a lighting means. It is already pointed out in the document EP 1 705 727 A1 that a certain proportion of blue or blue-green light always also has to be present to generate white light. However, the concept of phosphorescent emitters reaches its limits in this spectral region as far as it concerns the stability of the emitter molecules or even more of the matrix material, as the excitation energies approach the binding energies of the molecular components. Thus, the approach was chosen to some extent to cover the blue to blue-green spectral region with a fluorescent emitter and to cover the range comprising longer waves with phosphorescent emitters.
In the document EP 1 705 727 A1, a concept is described how, regardless of the quantum efficiency of direct light emission of a fluorescent blue emitter intrinsically restricted to 25%, the overall efficiency of a white-light OLED can be brought to 100% by using a fluorescent blue emitter with a triplet energy higher than the triplet energy of at least one phosphorescent emitter employed. By diffusion of the per se non-emitting triplet excitons by the blue-emitting layer to a further emission layer which contains the phosphorescent emitter and subsequent exothermic energy transfer, the triplet excitons of the blue emitter may also be used for the light emission.
An OLED with this concept can in fact in principle achieve a quantum efficiency of 100%; however, an experimental implementation shows that the quantum efficiency is greatly reduced, in particular at high light densities, due to the high exciton and charge carrier concentrations required for this. The long life span of the non-emitting triplet excitons in the blue-emitting layer is necessary, on the one hand, such that the excitons may diffuse up to the adjacent layer which contains the phosphorescent emitter. On the other hand, it leads to a high accumulation of the excitons in their main formation zone. This leads to an intensified non-emitting extinction of excitons as, with an increasing exciton density, an exciton may revert more easily to the basic state by losing its energy during another excitation of another exciton (exciton-exciton annihilation). The quantum efficiency thus decreases with an increasing current density which is generally known as a “roll-off” of the efficiency.
Conventional electrically undoped organic light-emitting diodes (OLEDs) which are also referred to as organic luminescent diodes may have a reduced number of layers depending on the design. Here, for example, functions of a hole transport layer and an electron and exciton blocking layer or a hole and exciton blocking layer and an electron transport layer may each be combined in an emission layer structure.
The production of the individual organic layers can be performed by means of thermal evaporation, molecular beam epitaxy, spin-on deposition from solutions and chemical vapour deposition. Customary methods, such as the evaporation of organic materials, only allow for the structuring in one dimension. The standard emission layer structure may consist of a mixed evaporation of a host or matrix material and the phosphorescent emitter dye, usually with concentrations between 1% by mole and 20% by mole.
A light-emitting component having organic layers and an emission of triplet excitons states with increased efficiency is described in the document DE 102 24 021 B4 in which the component consists of a layer order with a hole-injecting contact—the anode—, one or more hole-injecting and hole-transporting layers, a system of stacked layers in the light emission zone, one or more electron-injecting and electron-transporting layers and an electron-injecting contact—a cathode—, the light emission zone consisting of a series of hetero transitions which form boundary surfaces between the stacked layers, alternately a layer made of a material having hole-transporting or bipolar transport properties and a layer made of another material having electron-transporting or bipolar transport properties being arranged in the system of stacked layers and at least one of the materials being mixed with a triplet emitter dopant.
One problem is that an increase of the outer quantum efficiency can in fact be achieved with the stacking sequences of different mixed systems, however, no reduced decrease of the efficiency can be observed at high light densities in these structures. Additionally, the stacking sequences of the mixed systems lead to an increased recombination efficiency by capturing charge carriers on the boundary surfaces of the hetero transitions.
Phosphorescent light-emitting components are known, in particular also implemented as OLEDs. In known components, the outer quantum efficiency decreases markedly at high light intensities. The main cause of the loss in efficiency is the nature of phosphorescent emitter molecules which, in contrary to conventional fluorescent dyes, emit from the electronic triplet state. The actual quantum-mechanically prohibited transition with total spin one is accessible through the use of heavy metals, such as platinum or iridium, as the central atom for the light emission. The excited states, referred to as excitons, have a moderate life span which even for modern phosphorescent emitter molecules is longer by orders of magnitude than that of fluorescent dyes. For this reason, the triplet excitons are markedly more susceptible to any imaginable annihilation mechanism leading to the extinction of such an excited state, without the state being able to contribute to the emission. Therefore, the quantum efficiency decreases markedly at high excitation densities and light intensities, respectively.
The biggest developmental leap in the field of organic luminescent diodes in a thin-film OLED is described by Tang et al., Appl, Phys. Lett. 51 (1987) 913, in which the introduction of phosphorescent molecules as emitter molecules was provided (cf. Baldo et al., Nature 395 (1998) 151). They have the decisive advantage to allow for an internal conversion efficiency of 100%. By using heavy metals, such as platinum or iridium, as the central atom in these molecules, the electronic structure is influenced heavily which in this case results in that both the emission from the triplet state and the intermolecular transfer rate from singlet to triplet state (ISC—“intersystem crossing”) are enabled and can be highly efficient (cf. Tsuboi, Journ. Lumin. 119-120 (2006) 288).
In organic luminescent diodes, the charge carriers, namely electrons and holes, are injected statistically with regard to their spin such that this results in the excited states referred to as excitons are also formed with a statistical spin distribution. Due to the spin multiplicity from one to three in the case of the singlet and triplet states, on average only approx. 25% of the excitons are formed in the singlet state (cf. Baldo et al., Phys. Rev. B 60 (1999) 14422). This is in conventional fluorescent OLEDs the limit for the internal conversion efficiency, i.e. 25%. In phosphorescent molecules, both of the excitation states, singlet and triplet, are diverted to the triplet state (ISC) or fowled such that the molecules are potential materials for an internal quantum efficiency of 100% in OLEDs. One disadvantage of the molecules is the long life span of the excited state in comparison to fluorescent dyes. Even in the case of state-of-the-art phosphorescent molecules, the life span is longer by a few orders of magnitude, in the range of a few microseconds, cf. in this connection the emission from the singlet state with life spans of a few nanoseconds (Kawamura et al., J. Journ. Appl. Phys. 43 (2004) 7729).
In contrast to most fluorescent OLEDs, the emission zone of which representing a pure layer of the fluorescent dye, phosphorescent dyes are mixed into a host material in a diluted form to avoid a so-called aggregate extinction (cf. Kawamura et al., Phys. Rev. Lett. 96 (2006) 017404).
In this connection, it is important that the host material has a higher triplet energy than or at least a triplet energy as high as that of the emitter dye to exclude the energy transfer of the emitter triplet excitons to the excitons of the host molecules as a possible loss channel. Furthermore, the emission layers require relatively high concentrations of the emitter dye (˜5-10% by mole) as the energy transfer from the host material to the emitter molecule is slower and of a shorter range. The energy transfer is a Dexter process (cf. Dexter, Journ. Chem. Phys. 21 (1953) 836). This in turn is detrimental to the absolute efficiency as it can already come to the above-mentioned aggregate extinction with these concentrations. With this concentration, the direct exciton diffusion to the emitter molecules is possible without any problems.
Organic components having a mixed system of host material and phosphorescent emitter dye further contain blocker or blocking layers adjacent to the emission zone as the mean diffusion length of the excitons also increases with the long life span which should limit both charge carriers and triplet excitons to the emission zone. To achieve the latter, the layers need to have a significantly higher triplet energy than the emitter dye (Tblocker>>Temitter/˜0.4 eV) (cf. Goushi et al., Journ. Appl. Phys. 95 (2004) 7798).
The long life span renders the excited triplet states very susceptible to all the annihilation mechanisms, in particular at high excitation densities, the annihilation mechanisms being triplet-triplet annihilation (TTA), triplet-charge carrier annihilation and field-induced dissociation of the excitons in free charge carriers (cf. Reineke et al., Phys. Rev. B 75 (2007) 125328). In this connection, the triplet-triplet annihilation is the dominant process. En principle, the process may take place by means of two different mechanisms: (a) by diffusion of the triplet excitons until two excited states are close enough to another to annihilate,
and (b) by long-range interaction, based on the energy transfer model according to Förster.
If two excited states are that close that the electronic orbitals overlap and interact, the annihilation step may also take place by means of the so-called energy transfer according to Dexter. In the latter, the annihilation is performed in one step. The process is only dictated by the so-called Förster radius of the emitting molecule which defines the maximum distance of two excited states in which an annihilation still takes place (cf. Staroske et al., Phys. Rev. Lett. 98 (2007) 197402). The latter process is, in contrast to the diffusion-based mechanism, not dependent on the concentration of the emitter molecules, the Förster-based mechanism being an intrinsic boundary for the triplet-triplet annihilation TTA in the OLEDs as it is only dictated by the optical properties of the used materials. In other words, the diffusion-based annihilation mechanism is always intrinsically accompanied by the Förster-based annihilation.
To minimize the loss in efficiency in phosphorescent OLEDs, the approaches outlined below were chosen.
It was attempted to minimize the intrinsic life span of the excited triplet state on the emitter molecule. Here, it seems that the development is exploited; modern emitters have triplet life spans of a microsecond and less. The heavy metal influences the properties in a very significantly and its current optimum is found with iridium (cf. Baldo et al., Nature 395 (1998) 151; Baldo et al., Phys. Rev. B 62 (2000) 10967).
Furthermore, it was suggested to minimize the life span of the excited state by means of the optical properties defining the surroundings. By introducing a higher-quality OLED microcavity, the life span can be reduced by up to 50%, as described in Huang et al., Appl. Phys. Lett. 89 (2006) 263512.
Finally, it was proposed to minimize the exciton density which is required for a certain light intensity by means of widening the recombination zone in the emission zone. This can be achieved by means of the approach of a double emission layer, for example (He, Appl. Phys. Lett. 85 (2004) 3911).
In the case of organic light-emitting components, there are several examinations of the so-called emission layer, thus the region of the light generation, in which the emission zone is made up by quantum well structures, most of the times, however, by using fluorescent dyes. In this connection, there are efficiency improvements which are to some extent drastic, the main reason for which is that the recombination efficiency is increased, for example by improved capturing of charge carriers (Huang, Appl. Phys. Lett. 73 (1998) 3348; Xie, Appl. Phys. Lett. 80 (2002) 1477). As the loss in efficiency in fluorescent OLEDs due to the short life span of the singlet state is only marginally determined by bimolecular processes, no improvements with regard to the loss in efficiency are achieved by means of these structures.
Phosphorescent OLEDs, the emission zone of which having a quantum well structure, are described in Cheng et al., Jpn. J. Appl. Phys. 42 (2003) L376. Here, the known triplet emitter 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP) is used. Compared with a standard structure, an increase of the external quantum efficiency by a factor of about two is described for the quantum well structure. The quantum well structure has a stronger loss in quantum efficiency at higher light intensities such that the given structures ultimately have almost identical efficiency values at current densities of about 200 mA/cm2. The effect is explained by the saturation of the emitter molecules.
The spatial separation of different regions of mixed systems of host material and emitter dye has another advantage. The separation decouples largely any energy transfer, also and in particular at low excitation densities, between the centres. Typical phosphorescent emitters have Förster radii of less than 2 nm such that the Förster-based energy transfer between adjacent molecules can be prevented by distances in this order of magnitude (Kawamura et al., Phys. Rev. Lett. 96 (2006) 017404).
Emission layer structures in which all the emitter dyes are introduced into a host or matrix material in different concentrations were described (D'Andrade, Adv. Mat. 16 (2004) 624). The presented emission layer structure achieves about 16 μm/W at 100 cd/m2 and 11.1 μm/W at 1000 cd/m2. This corresponds to a reduction of the efficiency to 69% of the value at 100 cd/m2.